A polymer film (hereinafter referred to as film) has advantages such as excellent light transmission properties and flexibility, and is easy to be made lighter and thinner. Accordingly, the film is widely used as an optical functional film. In particular, a cellulose ester film using cellulose acylate or the like further has advantages such as toughness and low birefringence in addition to the above advantages. Therefore, the cellulose ester film is utilized as a photographic sensitive film, a protective film for a polarizing filter and an optical compensation film as components of a liquid crystal display device (LCD) whose market is increasingly expanded recently.
As a production method of the film described above, in general, solution casting method is used (as disclosed in Japanese Patent Laid-Open Publication No. 62-115035, for example). A film having more excellent properties such as optical properties can be produced in the solution casting method in comparison with other production methods such as a melt extrusion method. In the solution casting method, a polymer solution obtained by dissolving a polymer into various mixed solvents (hereinafter referred to dope) is discharged from a casting die and cast onto a casting drum to form a casting film. The casting film solidified on the casting drum is peeled as a wet film from the casting drum. Then, the wet film is dried to be wound as a film.
A film production apparatus utilizing the solution casting method houses the casting drum and the casting die in a casing in order to prevent solvent vapor which evaporates from the casting film and is harmful to humans from leaking outside. In a casing, there are provided a first labyrinth seal portion in an upstream side from the casting die in a rotational direction of the casting drum, and a second labyrinth seal portion in a downstream side from the casting die in a rotational direction of the casting drum (as disclosed in Japanese Patent Laid-Open Publication No. 2002-103360, for example). Accordingly, the casing is divided into a casting chamber containing the casting die and a drying chamber for drying a casting film formed on a peripheral surface of the casting drum during the transportation thereof.
A dry air supply duct for introducing dry air is provided in the drying chamber. The dry air introduced into the drying chamber by the dry air supply duct passes through the drying chamber, and exhausted from an exhaust port and a dry air exhaust duct. Thereby, the concentration of the solvent vapor in the drying chamber is kept low, and the drying of the casting film proceeds. Consequently, for the purposed of speeding up the film forming speed, the amount of dry air to be introduced into the drying chamber is increased.
Recently, due to rapid development of a liquid crystal display, a liquid crystal television, and the like, the demand for an optical film for use in the displays and the like has been increased rapidly. Therefore, it is necessary to speed up the film forming speed in order to increase productivity thereof. However, when the film forming speed is speeded up merely, there is a possibility in that a casting bead becomes unstable, and the surface quality of the obtained film is deteriorated. Moreover, in a serious case, the wet film may break and cause interruption and restart of the film production, thus decreasing the productivity.
Therefore, for the purpose of making the casting bead stable, a clearance of the discharge port of the dope in the casting die is made narrower toward the center thereof, and the thickness of the side ends of the casting bead is increased relatively in casting. However, when the thickness of the side ends of the casting bead is increased, the drying efficiency thereof is decreased due to the thickness, and the speed of gelation by cooling becomes slow. Therefore, gel strength corresponding to the peeling cannot be achieved, and in some cases, residue of the casting film which has not been peeled off is left on the casting drum. In this case, the surface of the film is deteriorated, and the breakage of the wet film occurs. Therefore, improvement has been desired.